Distinguishing TB Disease From Latent Infection

TB Life Cycle /Transmission
Tuberculosis (TB) is a disease caused by bacteria (Mycobacteria tuberculosis, or MTB) transmitted from people with active pulmonary or laryngeal TB disease as aerosolized particles that are suspended in air. Other people may inhale these infectious particles and become infected with MTB. In the vast majority of cases, their immune system responds to the infection and walls it off, resulting in a latent tuberculosis infection with no disease (and not contagious). When the infected person is immunosuppressed (e.g., HIV/AIDS) or has an immature immune system (young child), the infection may progress to primary active disease without latency.

Immune Response

The asymptomatic, noncontagious cases are identified by measuring their immune response to tuberculosis, using a TB skin test (TST) or a blood test (interferon gamma release assay, or IGRA).

Progression to Disease
About 5% of those with latent infection progress to active tuberculosis disease in the first 2 years after becoming infected. The risk for progression after that is about 0.1% per year (1% per decade of remaining lifetime). The risk of progression increases with conditions that suppress the immune system. This progression can occur with initial infection (primary disease) or after a latent period (reactivation disease). About 1 third of the world’s population has latent tuberculosis infection. People with evidence of TB infection and without active disease are often treated for latent infection to reduce the risk of progression to active disease.

Diagnosis of TB Disease
Active pulmonary tuberculosis is diagnosed with chest radiographs, looking for evidence of TB pneumonia and lymphadenopathy, and microbiological tests of sputum, which involve AFB stains, molecular tests, and cultures (with speciation by molecular tests of isolates). People with evidence of tuberculosis infection (positive TB skin test or IGRA) are given a chest radiograph to look for signs of TB pneumonia or fibrosis that would indicate current or prior TB disease, and if the chest x-ray is abnormal, sputum is tested for active disease.

TB Overview References

These CDC references provide an orientation to tuberculosis, diagnosis, treatment, and control. CME credit is currently available for the Core Curriculum and Self-Study Modules 1-5. Latent Tuberculosis Infection: A Guide for Primary Health Care Providers is a short, concise guide to the diagnosis and treatment of latent TB infection and is highly recommended for all clinicians who treat and/or test for latent TB or screen for active TB.